Rare earth-containing Y zeolite compositions

ABSTRACT

Y Zeolites are prepared with rare earth cations and Group VIII metal cations exchanged thereinto, such zeolites being especially useful in hydrocracking catalysts. The zeolites of the invention are prepared by exchanging a sodium Y zeolite with cations of one or more rare earth elements followed by a calcination, an ammonium ion exchange, and a Group VIII metal cation exchange. The resultant zeolite is not only highly active for catalytically promoting hydrocracking reactions but is also, after use in hydrocracking environments resulting in coke deposition, essentially completely regenerable by combustion of the coke.

BACKGROUND OF THE INVENTION

This invention relates to a catalytic hydrocracking process and aregenerable catalyst for use therein. More particularly, the inventionrelates to a hydrocracking catalyst, and most especially to noble metal,zeolitic hydrocracking catalysts, having highly improved regenerationproperties.

Petroleum refiners often produce desirable products, such as gasolineand turbine fuel, by catalytically hydrocracking high boilinghydrocarbons into product hydrocarbons of lower average molecular weightand boiling point. Hydrocracking is generally accomplished bycontacting, in an appropriate reactor vessel, a gas oil or otherhydrocarbon feedstock with a suitable hydrocracking catalyst underappropriate conditions, including an elevated temperature and anelevated pressure and the presence of hydrogen, such that a hydrocarbonproduct is obtained containing a substantial proportion of a desiredproduct boiling in a specified range, as for example, a gasoline boilingin the range of 185° to 420° F.

Oftentimes, hydrocracking is performed in conjunction withhydrotreating, usually by a method referred to as "integral operation."In this process, the hydrocarbon feedstock, usually a gas oil containinga substantial proportion of components boiling above a desired endpoint, as for example, 420° F. in the case of certain gasolines, isintroduced into a catalytic hydrotreating zone wherein, in the presenceof a suitable catalyst, such as a non-zeolitic, particulate catalystcomprising a Group VIII metal component and a Group VIB metal componenton a porous refractory oxide support most often composed of alumina, andunder suitable conditions, including an elevated temperature (e.g., 400°to 1000° F.) and an elevated pressure (e.g., 100 to 5000 p.s.i.a.) andwith hydrogen as a reactant, the organonitrogen components and theorganosulfur components contained in the feedstock are converted toammonia and hydrogen sulfide, respectively. Subsequently, the entireeffluent removed from the hydrotreating zone is treated in ahydrocracking zone maintained under suitable conditions of elevatedtemperature, pressure, and hydrogen partial pressure, and containing asuitable hydrocracking catalyst, such that a substantial conversion ofhigh boiling feed components to product components boiling below thedesired end point is obtained. Usually, the hydrotreating andhydrocracking zones in integral operation are maintained in separatereactor vessels, but, on occasion, it may be advantageous to employ asingle, downflow reactor vessel containing an upper bed of hydrotreatingcatalyst particles and a lower bed of hydrocracking particles. Examplesof integral operation may be found in U.S. Pat. Nos. 3,132,087,3,159,564, 3,655,551, and 4,040,944, all of which are hereinincorporated by reference.

In some integral operation refining processes, and especially thosedesigned to produce gasoline from the heavier gas oils, a relativelyhigh proportion of the product hydrocarbons obtained from integraloperation will have a boiling point above the desired end point. Forexample, in the production of a gasoline product boiling in the C₄ to420° F. range from a gas oil boiling entirely above 570° F., it mayoften be the case that as much 30 to 60% by volume of the productsobtained from integral operation boil above 420° F. To convert thesehigh boiling components to hydrocarbon components boiling below 420° F.,the petroleum refiner separates the 420° F.+ high boiling componentsfrom the other products obtained in integral operation, usually afterfirst removing ammonia by a water washing operation, ahydrogen-containing recycle gas by high pressure separation, and an H₂S-containing, C₁ to C₃ low BTU gas by low pressure separation. Theresultant denitrogenated and desulfurized liquid is then distilled intoa C₄ to 420° F. overhead gasoline product and a 420° F.+ unconvertedfraction. This bottom fraction is then subjected to furtherhydrocracking in a second hydrocracking zone wherein yet more conversionto the desired C₄ to 420° F. product takes place.

In the foregoing process, the two hydrocracking reaction zones oftencontain hydrocracking catalysts of the same composition. One catalystsuitable for such use is disclosed as Catalyst A in Example 16 of U.S.Pat. No. 3,897,327, herein incorporated by reference in its entirety,which catalyst is comprised of a palladium-exchanged stabilized Yzeolite hydrocracking component. But although the catalysts used in thetwo hydrocracking reaction zones may have the same composition and thesame catalytic properties, the hydrocracking conditions required in thesecond hydrocracking reaction zone are less severe than those requiredin the first. The reason for this is that ammonia is not present in thesecond hydrocracking reaction zone (due to water washing) whereas asignificant amount of ammonia is present in the first hydrocrackingzone. To account for the difference in operating conditions, it isbelieved that ammonia neutralizes or otherwise interferes with theacidity of the zeolite in the catalyst of the first reaction zone,thereby forcing the refiner to employ relatively severe conditions foroperation, as for example, increased temperature. On the other hand, inthe ammonia-deficient atmosphere of the second hydrocracking reactionzone, high conversions to the desired product are obtainable underrelatively moderate conditions, often with an operating temperatureabout 150° to 210° F. lower than that required in the firsthydrocracking reaction zone.

It has been discovered, however, that a difficult problem presentsitself when the hydrocracking catalyst in the second hydrocracking zonemust be regenerated. During hydrocracking, coke materials deposit on thecatalyst particles, and since the coke obviously interferes with theactivity of the catalyst, it is necessary to periodically regenerate thecatalyst by combustion of the coke. Curiously, however, it has beenfound that, after regeneration, the catalyst used in the secondhydrocracking reaction zone loses substantial activity for hydrocrackingunder the relatively moderate conditions employed therein. Even morecuriously, it has been found that, assuming identical catalysts are usedin the two hydrocracking zones, both remain useful after regenerationfor use in the first reaction zone, but both exhibit substantialactivity losses compared to fresh catalyst if used in the secondhydrocracking reaction zone.

Many attempts have been made to overcome the detrimental effectsassociated with regenerating hydrocracking catalysts for use in theammonia-deficient environments of the second hydrocracking zone, andparticularly with respect to catalysts containing noble metal-exchangedzeolites. But these attempts have largely focused on methods forrestoring some or all of the catalytic activity lost throughregeneration or other high temperature operation. These restoration (orrejuvenation) methods generally involve treating the regeneratedcatalyst or the coked catalyst prior to regeneration with an ammoniumsalt, ammonium hydroxide, gaseous ammonia, or mixtures thereof.Descriptions of typical prior art rejuvenation methods employingammoniated media may be found in U.S. Pat. Nos. 3,692,692, 3,835,028,3,849,293, 3,899,441, 3,943,051, 4,002,575, 4,107,031, 4,139,433, and4,190,553. The general theory behind these methods is that the activitylosses of catalysts used in hydrocracking environments are caused by theagglomeration of the otherwise dispersed Group VIII active metalhydrogenation component, and the ammoniation treatments of the prior artaim to reverse this mechanism and redisperse the Group VIII active metalcomponent.

Although the prior art methods have met with some success, a majordifficulty in their use is that the rejuvenation of the catalyst must beperformed under carefully controlled conditions in the presence ofammonia or an ammonium ion-containing medium, with all the attendantequipment and chemical costs associated therewith. Further, and far moreimportantly, by focusing on rejuvenation procedures, the prior art hasaimed at correcting a problem (catalyst deactivation) once it has comeinto existence rather than preventing the problem by providing acatalyst resistant to deactivation during regeneration. Further still,the prior art procedures are of only limited usefulness, beingapplicable, for example, to catalysts containing palladium-exchangedzeolites stabilized with magnesium cations but being of at most onlylimited usefulness with many other hydrocracking catalysts. As anillustration, hydrocracking catalysts containing hydrogen-palladiumzeolites have been found to be highly sensitive to ammonia or ammoniumion treatments, with collapse of the zeolitic crystal structure andvirtually complete loss of catalytic activity being the end result ofsuch treatment.

Accordingly, it is a major object of the invention to provide a zeolitichydrocracking catalyst resistant to deactivation during regeneration andother high temperature operations. It is a further object to provide aGroup VIII metal-exchanged zeolite, and particularly a noblemetal-exchanged zeolite, and a method for preparing such a zeolite,which is useful in a hydrocracking catalyst and resistant to losses incatalytic activity under high temperature conditions, particularly thehigh temperature oxidizing conditions that pertain during regenerationof hydrocracking catalysts. Yet another object of the invention is toprovide a noble metal-exchanged, zeolitic hydrocracking catalyst, and amethod for preparing such a catalyst, for use in a catalytichydrocracking process wherein high temperature regenerations of thecatalyst are periodically required without incurring substantial lossesin catalytic hydrocracking activity. A further object is to provide ahydrocracking process taking advantage of the regeneration-resistantproperties of the zeolite and hydrocracking catalyst of the invention.These and other objects of the invention will become more clear in lightof the following description of the invention.

SUMMARY OF THE INVENTION

The present invention is founded on the discovery that hydrocrackingcatalysts used in ammonia-deficient hydrocracking environments may bedramatically improved by utilizing as the zeolite component certain Ytype zeolites containing a rare earth metal and a Group VIII metal, andin particular certain Y type zeolites containing a rare earth metal anda noble metal. Accordingly, the invention is directed to a zeolite ofhighly improved properties, particularly with respect to resistinglosses in hydrocracking activity when regeneration by coke combustion isnecessary after service in ammonia-deficient hydrocracking environments.In its broadest embodiment, the zeolite of the invention is acomposition prepared by cation exchanging a zeolite of the Y type withcations of one or more rare earth metals followed by a calcination, anion exchange with ammonium ions, and an ion exchange with Group VIIImetal cations.

In one embodiment of the invention, especially useful in hydrocracking,the zeolite is prepared by cation exchanging a sodium Y zeolite withrare earth cations, steam calcining the resulting rare earth-exchangedzeolite, usually so as to effect at least some reduction in the zeoliteunit cell size, and subsequently subjecting the zeolite to an ammoniumion exchange and an exchange with noble metal cations. In anotherembodiment useful in hydrocracking, a zeolite is prepared by the cationexchange of a sodium Y zeolite with rare earth cations followed by (1) asteam calcination, usually so as to result in some shrinkage in thezeolite unit cell size, (2) an ammonium ion exchange, (3) an optionalcalcination, (4) a cation exchange of a noble metal into the zeolite,and (5) a final calcination.

Zeolites of the invention have been found to have catalytic activitywith respect to hydrocarbon conversion reactions, and for catalyticpurposes, the zeolite of the invention is usually admixed with arefractory oxide component, such as alumina in a hydrous gel form, priorto the final calcination. Catalysts of the invention are useful in avariety of hydrocarbon conversion processes, and especially inhydrocracking processes, with particular use being found in ahydrocracking process employing an ammonia-deficient environment. In onespecific embodiment of the invention, the hydrocracking catalyst of theinvention is employed in the second hydrocracking zone of a petroleumrefining process wherein a hydrocarbon feedstock is firstly treated in apetroleum refining process employing an integralhydrotreating-hydrocracking operation followed by further hydrocrackingof unconverted components in a second hydrocracking zone wherein anammonia-deficient environment is maintained.

BRIEF DESCRIPTION OF THE DRAWING

The drawing depicts, in flow sheet format, a preferred embodiment of theprocess of the invention for refining hydrocarbons, in which process acatalyst containing the novel zeolite of the invention is employed. Forthe sake of simplicity, many conventional items normally utilized inrefinery operation, such as pumps, valves, pressure gauges, andcompressors, whose description is not necessary to an understanding ofthe invention, have been omitted.

DETAILED DESCRIPTION OF THE INVENTION

The zeolite of the present invention may be prepared from a crystallinealuminosilicate zeolite of the Y type, the Y type zeolites being wellknown and described, for example, in U.S. Pat. No. 3,130,007, hereinincorporated by reference in its entirety. Most usually, the zeolite Ystarting material is in the sodium form, containing between about 10 and14% by weight sodium calculated as Na₂ O. In addition, the Y zeolitestarting material will have a silica-to-alumina ratio above 3, and mostusually and preferably between about 3 and 6.

In accordance with this invention, the Y zeolite starting material ispartially ion-exchanged with rare earth metal cations. If, as ispreferred, the starting material is a sodium Y zeolite, the ion exchangeis such that the resultant zeolite contains a rare earth metal and atleast some sodium, but usually less than about 5% by weight, calculatedas Na₂ O. The rare earth metals selected for exchange into the zeolitemay be any one or any combination of the lanthanide elements havingatomic numbers according to the Periodic Table of Elements between 57and 71. Thus, the metals suitable for ion exchange herein includelanthanum, cerium, praeseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium. In the most preferred embodiment of the invention, amixture of rare earth cations is introduced into the zeolite, with themixture often containing rare earth metals in a distribution similar tothat of the rare earth ore (e.g., bastnaesite, monazite, xenotime, andthe like) from which the metals were derived.

There are many known methods by which one can exchange rare earthcations for sodium and other cations in a crystalline aluminosilicatezeolite. The most usual way is to contact the zeolite with an aqueoussolution containing multivalent cations of the rare earth element orelements to be exchanged into the zeolite. Most often, the solution willcontain more than about 20 grams per liter of rare earth metal cations(calculated as RE₂ O₃ where RE is the sum of all rare earth metals underconsideration, regardless of whether any one or more of such metalsactually forms a trioxide of equivalent formula), and the contacting isusually accomplished by immersing the zeolite into the ion-exchangesolution and heating the resultant solid-liquid mixture, with stirring,to a temperature above ambient but usually to no more than about 100° C.If desired, the solution may also contain ammonium ions, and thesolution may further contain any of a number of anions that will notinterfere with the cation exchange, e.g., chloride, nitrate, sulfate,etc.

For best results, the ion exchange is performed in a manner such thatthe resulting zeolite contains less than about 5% by weight of sodium,usually between 1 and 5% by weight of sodium, calculated as Na₂ O, andusually at least about 2%, preferably at least about 5%, by weight ofrare earth metals, calculated as RE₂ O₃. Normally, only a singleimmersion of the zeolite into the ion exchange solution will besufficient for the necessary exchange. However, it is well within thescope of the invention to carry out the ion exchange by severalimmersions into a solution containing rare earth metal cations, or byimmersion serially into several solutions of differing rare earthelement content, or by other known methods for introducing rare earthmetal cations into a zeolite.

After ion exchange, the rare earth-containing Y zeolite, which isusually and preferably a sodium and rare earth-containing zeolite Y, issubjected to a calcination treatment. The calcination treatment ispreferably accomplished in the presence of steam and may be termed asteam calcination. The steam calcination is accomplished by heating thezeolite in the presence of water vapor to at least about 600° F.,usually between about 900° and 1600° F., and preferably in the range ofabout 1100° to 1350° F. The water vapor partial pressure during thesteam calcination is generally above about 0.2 p.s.i.a., usually above1.0 p.s.i.a., preferably between 2 and 15 p.s.i.a., and more preferablystill between 5 and 15 p.s.i.a. In the most preferred embodiment, thesteam calcination is performed in the presence of a gaseous atmosphereconsisting essentially of water vapor.

The steam calcination is generally carried out for a time periodcorrelated with the severity of the calcination conditions, especiallythe water vapor partial pressure and calcination temperature, so as toeffect at least some reduction in the unit cell size of the zeolite butwithout resulting in substantial degradation of the zeolite crystalstructure. The usual unit cell size of the zeolite prior to steamcalcination is between about 24.65 and 24.75 Å and after steamcalcination is most often between about 24.20 and 24.64 Å, preferablybetween about 24.40 and 24.64 Å. However, steam calcination conditionsknown to be effective in the invention, regardless of resultant unitcell size, are a calcination temperature of 1100° to 1350° F. maintainedfor one to two hours at essentially atmospheric pressure in the presenceof water vapor at a partial pressure above about 12 p.s.i.a. Theseconditions are particularly effective with respect to a sodium Y zeoliteconverted to the rare earth exchanged form by slurrying the zeolite inan aqueous solution containing 35 to 120 mg. per ml. of total dissolvedrare earth elements, calculated as the individual metals. In thisembodiment of the invention, the slurry will usually comprise one gramof zeolite per 3 to 4 milliliters of solution.

It will be found that the steam calcination treatment may be effectuatedby any of a number of procedures. In one method, the wet zeolite fromthe rare earth exchange step is merely heated in an enclosed vesselpreventing the escape of water vapor generated therein. Alternatively,the wet zeolite, with or without added water, may be heated in anautoclave equipped with a pressure relief valve such thatsuper-atmospheric pressures of steam can be maintained therein.Alternatively still, the zeolite may be introduced into a batch orcontinuous rotary furnace, or a static bed calcination zone, into whichpreheated steam or humidified air is introduced. For commercialpurposes, it is expected that the most useful and convenient method forsteam calcining the zeolite is by introduction of preheated steam into acontinuously rotating furnace.

Subsequent to the steam calcination, the zeolite is ion-exchanged withammonium ions to lower the sodium content of the zeolite still further,usually until the sodium content is less than about 0.5% by weight,preferably less than 0.2% by weight, and most preferably less than about0.05% by weight, calculated as Na₂ O. Ordinarily, the ion exchange issuch that at least 25%, often at least 50%, of the residual zeoliticsodium ions in the zeolite are exchanged with ammonium ions.

The ammonium ion exchange may be accomplished by methods analogous tothose described hereinbefore with respect to the rare earth cationexchange. That is, the ammonium ion exchange may be accomplished bycontacting the zeolite with an ammonium ion-containing solution.Usually, the zeolite is stirred for one to five hours at temperaturesabove ambient but less than about 100° C. in an aqueous solutioncontaining a dissolved ammonium salt, as for example, ammonium nitrate,ammonium sulfate, ammonium chloride, and the like. Ordinarily, toachieve extremely low sodium levels in the final zeolite, it will provenecessary to repeat the ion exchange procedure at least once if sodiumlevels below about 0.05 weight percent (calculated as Na₂ O) aredesired. More often, the ion exchange procedure will be repeated atleast twice, and occasionally several times, before reductions in sodiumcontent below 0.05 weight percent are achieved.

After the ammonium ion exchange, the zeolite is separated from the ionexchange solution, washed free of any residual ion exchange solution,and then heated at 100° to 200° C. for a time period sufficient toproduce a dried product. Usually, time periods of one to two hours proveeffective.

The dried zeolite product containing ammonium cations and rare earthmetal cations is useful as a molecular sieve, as for example indehydrating gases containing water vapor or for separating normalparaffins from isoparaffins. The zeolite product is also useful in thecatalysis of hydrocarbon conversion reactions, especially with respectto acid catalyzed hydrocarbon conversion reactions, such as cracking,alkylation, isomerization, etc., or for acid catalyzed reactions coupledwith hydrogenative reactions, such as hydrocracking, hydroisomerization,and the like. For cracking, alkylation, and other non-hydrogenativereactions, the zeolite is ordinarily dispersed in a porous refractoryoxide matrix usually composed of alumina, silica, magnesia, beryllia,zirconia, titania, thoria, chromia, or combinations thereof. Also usefulare silica-alumina, silica-zirconia, and the like. For hydrocracking andother combined hydrogenative-acid catalyzed reactions, one or morehydrogenation components are further added, with Group VIB and VIIImetals, often in combination, being utilized for this purpose.

When used for catalytic hydrocarbon conversion purposes, the zeolite ofthe invention is, at some point in the catalyst preparation procedure,calcined at an elevated temperature, usually in the 600° to 1600° F.range, preferably in the 900° to 1500° F. range. This calcinationprocedure may be applied to the dried zeolite product prior to admixturewith other catalytic materials or, as is more often the case, to thezeolite when further combined with a precursor of the desired refractoryoxide (for instance, alumina gel in the case of alumina, silica gel inthe case of silica, etc.), or with a salt containing the one or moredesired hydrogenation metals, or with both. Calcination after admixturewith other catalytic materials serves several purposes at once. Inaddition to converting the zeolite of the invention to a form moreactive for acid catalyzed hydrocarbon conversion reactions by thedecomposition of the ammonium ions to hydrogen ions and hydroxyl groups,calcination will further convert the refractory oxide precursor to itsdesired form, e.g., gamma alumina, and the hydrogenation metal salt tothe corresponding metal oxide.

The zeolites of the present invention, although useful in a wide varietyof hydrocarbon conversion catalysts, find especial usefulness inhydrocracking catalysts. As stated hereinbefore, the typicalhydrocracking catalyst of the invention contains one or morehydrogenation metal components, a porous refractory oxide, and thezeolite of the invention. Ordinarily, the hydrogenation metal chosen isa Group VIII metal, with at least some of said Group VIII metal beingincorporated into the zeolite by cation exchange after the ammonium ionexchange or after a calcination subsequent to the ammonium ion exchange.If desired, a Group VIB metal, and particularly molybdenum, may also beion-exchanged into the zeolite, as for example by the method disclosedin U.S. Pat. No. 4,297,243, herein incorporated by reference. Morecommonly, however, if a Group VIB metal is utilized, it is usuallyintroduced after the zeolite is admixed with a refractory oxidecomponent, the usual procedure being to calcine the admixture,impregnate with a solution containing the Group VIB metal in an anionicform, such as ammonium heptamolybdate, and calcining again. Normally,the Group VIII metal will be introduced by cation exchange prior toadmixture with the refractory oxide component, but, if desired one mayintroduce the Group VIII metal in cationic form into the zeolite byimpregnation at a time subsequent to admixing the zeolite and refractoryoxide but prior to the final calcination.

The most preferred zeolite of the invention for use in hydrocrackingcatalysts contains a noble metal as the Group VIII hydrogenation metal.The noble metals are selected from the group consisting of platinum,palladium, rhodium, iridium, ruthenium, and osmium, but the preferrednoble metals are platinum and palladium, with palladium being mostpreferred. The noble metal, or combination of noble metals, is dispersedin the zeolite by cation exchange, as for example, by contacting thedried rare earth-ammonium zeolite described above with an aqueoussolution having platinum dichloride or palladium dichloride dissolvedtherein. To facilitate the ion exchange, ammonium ions are usually alsointroduced into the aqueous solution, as by the addition of ammoniumhydroxide. The ion exchange is usually such as to introduce at leastabout 0.1% by weight, preferably between 0.1 and 2.0% by weight of noblemetal, calculated as the metal, into the zeolite, which zeolite, fromprevious ion exchanges, contains one or more rare earth metals in aproportion usually above about 2 percent by weight, preferably aboveabout 5 percent by weight, calculated as RE₂ O₃, and further containingsodium in very small proportions, usually below about 0.5 percent byweight, preferably below about 0.05 percent by weight.

After incorporation of the noble metal into the zeolite by the foregoingor equivalent methods, the zeolite is combined with a refractory oxidecomponent. For example, the noble metal-rare earth-ammonium zeolite maybe admixed with peptized alumina, alumina gel, or hydrated alumina,usually in amounts such that the zeolite comprises between 50 and 90% byweight of the admixture. The admixture is then formed into particulates,as by extrusion through a die having openings of a cross-sectional sizeand shape desired in the final catalyst particles followed by incisionof the extruded matter into lengths of about 1/16 to 1/2 inch. Theresulting particulates are then subjected to a calcination at anelevated temperature, usually between about 600° and 1600° F., toproduce catalytic particles of high crushing strength.

Other methods for producing a particulate hydrocracking catalystcontaining a noble metal dispersed in the zeolite are also applicable.For example, the dried rare earth-ammonium Y zeolite describedhereinabove may be admixed with a refractory oxide component, such asalumina gel, and the resulting admixture may then be calcined, followedby impregnation with an ammonical solution containing platinumpalladium, or other noble metal in a cationic species. Such animpregnation, while distributing some of the noble metal upon the porousrefractory oxide, also results in the introduction of noble metal intothe ion exchange sites of the zeolite, and a subsequent calcinationconverts the exchanged noble metal into the oxide form. Alternativelystill, the zeolite may be admixed with a solid noble metal salt (againcontaining the noble metal in a cationic species), a porous refractoryoxide precursor, and sufficient water containing ammonium hydroxide intoa paste suitable for extrusion or molding into the desired particulatesize and shape. During the mixing and extruding (or molding) operations,at least some of the noble metal will exchange into the zeolite andbecome dispersed therein.

Noble metal hydrocracking catalysts prepared by the foregoing oressentially equivalent procedures are characterized by the presence ofone or more noble metals on a support comprising a porous refractoryoxide plus a Y-type zeolite containing hydrogen cations and one or morerare earth metals, with at least some of said noble metal beingcontained within the zeolite. The most preferred catalyst consistsessentially of alumina combined with a palladium-containing, rareearth-containing, and hydrogen ion-containing zeolite, with the zeolitehaving been prepared, as described above, by a method including thesteps of rare earth cation exchange followed by a steam calcination andsubsequent ammonium ion and noble metal cation exchanges.

Hydrocracking catalysts prepared with the zeolite of the invention areuseful in the conversion of a wide variety of hydrocarbon feedstocks toa hydrocarbon product of lower average boiling point and molecularweight. The feedstocks that may be subjected to hydrocracking by themethod of the invention include all mineral oils and synthetic oils(e.g., shale oil, tar sand products, etc.) and fractions thereof.Illustrative feedstocks include straight run gas oils, vacuum gas oils,and catcracker distillates. The typical hydrocracking feedstock,however, contains a substantial proportion of components, usually atleast 50% by volume, often at least 75% by volume, boiling above thedesired end point of the product, which end point, in the case ofgasoline, will generally be in the range of about 380° to 420° F.Usually, the feedstock will also contain gas oil components boilingabove 550° F., with highly useful results being achievable with feedscontaining at least 30% by volume of components boiling between 600° and1000° F.

For best results in hydrocracking, the catalyst of the invention will beemployed as a bed of catalytic particulates in a hydrocracking reactorvessel into which hydrogen and the feedstock are introduced and passedin a downwardly direction. Operating conditions in the reactor vesselare chosen so as to convert the feedstock into the desired product,which, in the preferred embodiment, is a hydrocarbon product containinga substantial proportion of gasoline components boiling, for example, inthe 185° to 420° F. range. However, other products, such as turbine fuelor diesel fuel, may also be desired on occasion, and conditions must beadjusted according to the product (or distribution of products) desired.The exact conditions required in a given situation will depend upon thenature of the feedstock, the particular catalyst composition utilized,and the desired product(s). But in general, the conditions of operationwill fall into the following suitable and preferred ranges:

                  TABLE I                                                         ______________________________________                                        Conditions      Suitable  Preferred                                           ______________________________________                                        Temperature, °F.                                                                       450-850   500-800                                             Pressure, psig   750-3500 1000-3000                                           LHSV            0.3-5.0   0.5-3.0                                             H.sub.2 /Oil, MSCF/bbl                                                                         1-10     2-8                                                 as measured at                                                                60° F. and                                                             1 atmosphere                                                                  ______________________________________                                    

The noble metal hydrocracking catalyst of the invention will findespecial use in a hydrocracking reactor vessel wherein an ammoniadeficient atmosphere is maintained. In the usual instance, anessentially nitrogen-free fluid, comprising hydrogen plus adenitrogenated hydrocarbon feedstock containing hydrocarbon componentsboiling above a maximum desired temperature, as for example, above 420°F. when a gasoline product boiling below about 420° F. is required, iscontacted with the noble metal catalyst of the invention underhydrocracking conditions converting a substantial proportion of the 420°F.+ hydrocarbon components to product components boiling in the desiredgasoline range. Under such ammonia-deficient conditions, it has beenfound that prior art catalysts comprising a noble metal-exchangedzeolite cracking component lose substantial activity after coke andcoke-like materials, which deposit on the catalyst under hydrocrackingconditions, are removed therefrom by a combustive regenerationtreatment. It is believed that such activity losses are due to themigration and subsequent agglomeration of the active noble metals, butwhatever the chemical mechanism by which such losses occur in the priorart catalysts, the catalyst of the present invention comprising azeolite containing an exchanged rare earth metal and a noble metalresists deactivation and is regenerable by combustion of coke depositswithout encountering activity losses.

It is, therefore, a specific embodiment of the invention to employ thehydrocracking catalyst of the invention, and particularly the noblemetal hydrocracking catalyst of the invention, in a hydrocrackingreactor vessel wherein a hydrocarbon feedstock is treated underammonia-deficient and preferably essentially ammonia-free hydrocrackingconditions. By ammonia-deficient, it is intended to mean the presence ofno more than 200 ppmv ammonia in contact with the catalyst, and byessentially ammonia-free, it is intended to mean the presence of no morethan 20 ppmv ammonia in contact with the catalyst. In bothammonia-deficient and essentially ammonia-free hydrocrackingenvironments, and also in essentially completely ammonia-freeenvironments wherein the ammonia content in contact with the catalyst ispresent in only trace amounts not exceeding 3 ppmv, the catalyst of theinvention proves to be a regenerable catalyst resistant to hydrocrackingactivity losses when coke deposits are removed from the catalyst bycombustion.

Accordingly, it is yet another specific embodiment of the invention toemploy the hydrocracking catalyst of the invention in a refining processwherein a hydrocarbon feedstock is treated by an integralhydrotreating-hydrocracking operation followed by further hydrocrackingof unconverted hydrocarbon components in an ammonia-deficienthydrocracking zone. The hydrocracking catalyst of the invention isemployed in the ammonia-deficient hydrocracking zone, and also, ifdesired, in the first hydrocracking zone utilized in integral operation.In this refining process, which is shown schematically in the flowdiagram of the drawing, the hydrocarbon feedstock carried in conduit 1is generally a gas oil boiling above about 550° F., usually in the rangeof 550° to 1200° F. After blending with a hydrogen-containing gas fromconduit 2, the feedstock-hydrogen blend is then passed by conduit 3 intoa suitable reactor vessel 4 wherein the feedstock is subjected tocatalytic hydrotreating in hydrotreating zone 5 maintained underconditions falling in the following ranges:

                  TABLE II                                                        ______________________________________                                        Operating Conditions                                                                           Suitable   Preferred                                         ______________________________________                                        Temperature, °F.                                                                        400-1,000  650-800                                           Pressure, p.s.i.a.                                                                             100-5,000  500-2,000                                         Space Velocity, LHSV                                                                           0.1-15     2-7                                               Hydrogen Recycle Rate,                                                                         400-20,000 4,000-10,000                                      cf/bbl as measured at                                                         60° F. and 1 atmosphere                                                ______________________________________                                    

The catalyst utilized to promote the hydrotreating reactions isgenerally composed of a Group VIII hydrogenation metal component incombination with a Group VIB hydrogenation metal component, oftentimesin conjunction with an additional acid component, such as phosphorus,supported on an amorphous, porous refractory oxide support such asalumina. A preferred hydrotreating catalyst comprises a sulfided,particulate composition comprising a nickel or cobalt component,preferably a nickel component in a proportion between about 2 and 6percent by weight (calculated as NiO), a molybdenum or tungstencomponent, preferably a molybdenum component in a proportion betweenabout 12 and 30 percent by weight (calculated as MoO₃), and a phosphoruscomponent present in a proportion between about 2 and 6 percent byweight (calculated as P) on a support consisting essentially of aluminaor alumina in combination with a small amount of silica. The catalyst isgenerally employed as a bed of particulates through which the feedstockand hydrogen are passed downwardly under conditions usually selectedfrom those shown in Table II so as to convert the organonitrogencomponents in the feedstock to ammonia and the organosulfur componentsto hydrogen sulfide. All the products recovered from the hydrotreatingstage are passed through a catalytic hydrocracking zone 6, which zonemay, as shown in the drawing, be maintained in the lower portion of thesame vessel containing the hydrocracking catalyst. Alternatively, thehydrocracking zone may be maintained in an entirely different reactorvessel. In either event, the entire effluent from the hydrotreating zoneis passed through a bed of hydrocracking catalyst particles underconditions usually selected from those shown in Table I so as to converta specified percentage of the hydrotreated feedstock, as for example,60% by volume, to products boiling below a specified boiling end point,400° F. being illustrative for many gasolines. The catalyst employed inhydrocracking zone 6 may be either the catalyst of the invention or aconventional catalyst, but if a conventional catalyst is chosen, it ispreferred that it be a catalyst containing a noble metal-exchanged,stabilized Y zeolite, such as that designated as Catalyst A in Example16 of U.S. Pat. No. 3,897,327.

It is noted that hydrocracking zone 6, due to the heat generated by theexothermic reactions occurring therein, is provided with ahydrogen-containing quench gas supplied through lines 7 and 8 fromheader 9. It is further noted that, in the preferred embodiment, theoperating pressures in hydrocracking zone 6 and hydrotreating zone 5 aresubstantially the same, whether the two zones are maintained in the samereactor vessel as shown in the drawing or in separate reactor vessels.Operating in this manner obviates pressure-reducing orpressure-increasing equipment between the hydrotreating andhydrocracking zones.

After hydrocracking, ammonia produced in hydrotreating zone 5 andcarried through hydrocracking zone 6 is separated from the otherhydrocrackate products recovered in conduit 10. This is accomplished bycombining the hydrocrackate products carried in conduit 10 with thoseobtained via conduit 33 from a second hydrocracking zone maintained inhydrocracking vessel 25, hereinafter discussed in fuller detail, andthen passing the resultant mixture by conduit 11 to appropriatefacilities for absorbing ammonia by a water-washing operation 12utilized in conjunction with high pressure separation. Water isintroduced by conduit 13, and sour water, containing essentially all theammonia originally present in the hydrocrackate products and some of thehydrogen sulfide which was also originally present therein, is removedby conduit 14.

Also recovered from the water-washing operation are a hydrocarbon liquidand a hydrocarbon recycle gas containing hydrogen, hydrogen sulfide, andlight hydrocarbon gases. These products are collected respectively inconduits 15 and 16, with the hydrocarbon liquid being then transferredto low pressure separator 17. From separator 17, a low BTU gascontaining C₁ to C₃ hydrocarbons plus essentially all the hydrogensulfide carried into separator 17 by conduit 15, is recovered by conduit18 while a liquid product, typically boiling at temperatures at or abovethat of C₄ hydrocarbons, is obtained in conduit 19. This C₄ + liquidproduct, which is essentially free of nitrogen and sulfur components, isintroduced into distillation column 20 and therein separated into agasoline product, boiling, for example, in the C₄ to 400° F. range, andan unconverted bottom fraction boiling primarily above 400° F. (or otherdesired end point of the gasoline product). From distillation column 20,the gasoline product is recovered by conduit 21 while the unconvertedbottom fraction is recovered by conduit 22, the latter then beingblended by conduit 23 with a portion of the hydrogen recycle gas carriedin conduit 16, with the resulting blend then being passed on by conduit24 to hydrocracking vessel 25. Also introduced into hydrocracking vessel25 is a hydrogen quench gas from header 9 by conduits 26, 27, 28, and29, which hydrogen quench gas is supplied by blending the recycle gasfrom conduits 16 and 30 with hydrogen make-up from conduit 31. Theblending takes place in conduit 32, which serves as a common source ofadded hydrogen for reactor vessels 4 and 25.

In hydrocracking vessel 25 is maintained a second hydrocracking reactionzone utilizing the hydrocracking catalyst of the invention andpreferably the noble metal catalyst of the invention. In this secondhydrocracking reaction zone, ammonia-deficient conditions, andpreferably essentially ammonia-free, and more preferably still,essentially completely ammonia-free hydrocracking conditions, aremaintained. Due to the reduced ammonia content in comparison to thefirst hydrocracking reaction zone 6 (wherein ammonia concentrations of2000 ppmv or higher are usually maintained), the hydrocrackingconditions employed in hydrocracking vessel 25 are less severe, andsubstantially lower operating temperatures may be utilized whileachieving high conversions (e.g., 60 volume percent or better) toproducts boiling below the desired end point. Thus, the conditions foroperation in the second hydrocracking reaction zone may be selected fromthose shown in Table I but will, overall, be less severe than thoserequired in the hydrocracking zone associated with integral operation.Usually, the operating temperature in hydrocracking vessel 25 is betweenabout 450° and 600° F. whereas the operating temperature inhydrocracking zone 6 is higher, generally between about 600° and 850° F.

It should be noted, however, that although the conditions inhydrocracking reactor vessel 25 are preferably ammonia-deficient at alltimes, one can, in less preferred embodiments of the invention, maintainreactor vessel 25 under ammonia-rich conditions, for example, underconditions similar to those maintained in hydrocracking zone 6. Suchoperation, of course, requires conditions of increased severity, and forthat reason, ammonia-rich conditions in reactor vessel 25 are generallyavoided. But there will be many instances when, even under plannedammonia-deficient conditions, a petroleum refiner may temporarilyrequire the use of ammonia-rich conditions in reactor 25. Generally,however, such conditions will be permitted for only short periods oftime, and in the most usual situation, the ammonia content in reactorvessel 25 over a 30-day time period will average less than 200 ppmv,i.e., the operation will be under average ammonia-deficient conditions.

COMPARATIVE EXAMPLE

This Example demonstrates the hydrocracking activities of the catalystof the invention and a comparative catalyst in ammonia-deficienthydrocracking environments. The activities are compared both before andafter the catalysts are regenerated by combustion of coke.

The comparative catalyst is composed of 80% zeolite and 20% alumina,with sufficient palladium exchanged into the zeolite such that thecatalyst composition contains 0.54% by weight palladium (calculated asthe metal). The catalyst is prepared in a manner similar to that ofCatalyst A described in Example 16 of U.S. Pat. Nos. 3,897,327 and3,929,672, both herein incorporated by reference in their entirety. Thecatalyst has a compacted bulk density of 0.64 gm/cc. and is in the formof 1/8 inch diameter cylindrical extrudates.

A catalyst of the present invention is then prepared in accordance withthe following procedure: 180 gm. of rare earth chloride is dissolved in150 ml of deionized water. The rare earth chloride contains a mixture ofrare earth elements in the following proportions calculated as theoxides: 50 weight percent CeO₂, 33 weight percent La₂ O₃, 12 weightpercent Nd₂ O₃, 4 weight percent Pr₆ O₁₁, and 1 weight percent otherrare earth elements, calculated as RE₂ O₃. The resulting rare earthsolution is filtered to remove insoluble materials, and 130 ml of thefiltrate is then added to a suspension of 500 gm. sodium Y zeolite in1500 ml of water. The slurry thus formed is heated to about 80° to 90°C. with stirring for one hour, after which the zeolite is separated fromthe slurry by filtration and washed free of chloride with deionizedwater. Two hundred-eighty grams of the resultant rare earth-exchangedzeolite, containing 6.9 percent by weight of rare earth oxides(calculated as RE₂ O₃, is then heated in a preheated furnace at 670° C.for one and one-half hours in the presence of flowing steam. The steamedproduct is then ion-exchanged with ammonium ions by immersion for threehours at 80° to 90° C. in a solution composed of 200 gm. ammoniumnitrate dissolved in 500 ml deionized water. This procedure is repeatedtwice more, after which the ammonium-exchanged zeolite is recovered byfiltration, washed with deionized water to remove all trace of nitrates,and then subjected to a temperature of 110° C. for a time periodsufficient to produce a dried product containing about 0.03% by weightsodium (calculated as Na₂ O) and having a unit cell size of 24.577 Å.The zeolite is then ion-exchanged with palladium by first dissolving2.30 grams of palladium dichloride in a liquid composed of 30 ml ofconcentrated (33%) ammonium hydroxide and 200 ml water and then addingall of the resultant Pd-containing solution, in dropwise fashion andwith stirring, to a suspension of 200 grams of zeolite in a solutioncontaining 500 ml of deionized water and 25 ml of concentrated ammoniumhydroxide. After standing overnight, the suspension (or slurry) isfiltered to yield a palladium-containing zeolite, which is washed withdeionized water and dried. This dried product is then admixed withpeptized Catapal™ alumina, with the proportion of alumina in theadmixture being equal to 20% of the weight of the zeolite. Afterextrusion through a die having 1/16 inch circular openings therein, theextruded matter is cut into 1/8 to 1/2 inch lengths and calcined inflowing air at 800° F. for one hour. The resulting catalyst has acompacted bulk density of 0.59 gm/cc, a surface area of 645 m² /gm, asodium content (as Na₂ O) of 0.031% by weight, a rare earth metalcontent of 5.01% by weight (calculated as RE₂ O₃), and a palladiumcontent of 0.57% by weight, calculated as the metal. The zeolitecontained in the catalyst is found to have a unit cell size of 24.553 Å.

The catalysts are then evaluated for hydrocracking activity in twoseparate runs wherein a gas oil feed plus added hydrogen is passedthrough a laboratory-sized reactor vessel containing 150 cc. of catalystunder the following conditions: 1450 p.s.i.a., 1.7 LHSV, and ahydrogen-to-oil ratio of 8000 SCF/bbl. The gas oil feed is adenitrogenated, desulfurized, unconverted fraction obtained from aprevious integral hydrofining-hydrocracking operation; it has an APIgravity of 38° and a boiling range of 360° to 870° F., with about 12percent by volume of the feed boiling below 400° F. To simulatehydrocracking in an H₂ S-containing atmosphere, thiophene is blendedwith the feedstock so as to provide a sulfur concentration therein of0.5 weight percent. The operating temperature utilized in the reactorvessel is adjusted periodically to maintain a total liquid productgravity of 49.5° API, which, by previously established correlations,corresponds to a 60 volume percent conversion of the feedstock to a C₄to 400° F. gasoline product. The temperatures required to maintain thisconversion after 100 hours on stream are 519° F. for the catalyst of theinvention and 512° F. for the comparative catalyst, which data indicatethat the two catalysts have roughly equivalent hydrocracking activityprior to regeneration.

To provide data relative to the activities of the catalysts afterregeneration, the catalysts are each coked at the end of theirindividual runs. The coking is accomplished by changing thehydrocracking conditions from ammonia-deficient to ammonia-rich, therebyrequiring more severe conditions to produce the desired C₄ to 400° F.gasoline product. The ammonia-deficient conditions are changed toammonia-rich conditions by introducing into the hydrocracking reactorvessel tert-butyl amine at a rate equivalent to about 1 gram of nitrogenper 200 grams of feed. The reactor temperature is then increased(without changing other conditions) so as to produce and maintain therequisite 60 volume percent conversion to a C₄ to 400° F. gasolineproduct over about a 125 to 150 hour time period, at the end of whichthe catalysts are in a coked condition.

After coking, the catalysts are regenerated by a controlled oxidativecombustion at temperatures ranging from about 700° to 1000° F. utilizinga flowing regeneration gas consisting essentially of nitrogen andoxygen. The oxygen content of the regeneration gas is increased from 0.1to 3.0 volume percent as necessary to maintain a water vapor partialpressure in the gaseous combustion products at values below about 0.25p.s.i.a. The regenerated catalyst particles are then activity-tested inaccordance with the activity test described above, and the catalyst ofthe invention is found useful for producing the required C₄ to 400° F.gasoline product at an operating temperature of 510° F. in comparison to540° F. for the comparative catalyst. These data establish that thecatalyst of the invention is substantially more active afterregeneration than is the case for the comparative catalyst. Indeed, asshown by the following Table III summarizing the data obtained in theforegoing experiments:

                  TABLE III                                                       ______________________________________                                                        Temperature Required                                          Catalyst        for 60% Conversion                                            ______________________________________                                        Comparative Catalyst                                                                          512° F.                                                Prior to Regeneration                                                         Comparative Catalyst                                                                          540° F.                                                After Regeneration                                                            Catalyst of Invention                                                                         519° F.                                                Prior to Regeneration                                                         Catalyst of Invention                                                                         510° F.                                                After Regeneration                                                            ______________________________________                                    

it will be seen that the catalyst of the invention actually exhibited a9° F. increase in catalytic activity during regeneration while thecomparative catalyst demonstrated a 28° F. drop in catalytic activity.And the 30° F. differential between the catalyst of the invention andthe comparative catalyst after regeneration indicates that the catalystof the invention is more than twice as active after regeneration as thecomparative catalyst. For example, the catalyst of the invention couldbe used under the same conditions as the comparative catalyst andproduce similar results while treating the same feedstock but at morethan twice the space velocity.

It should also be noted in the experiment of the Comparative Examplethat the catalyst of the invention and the comparative catalyst wereboth coked under ammonia-rich conditions. And since the data afterregeneration indicate that the catalyst of the invention is highlyactive, indeed even more so than the comparative catalyst, it is evidentthat the catalyst of the invention resists catalytic deactivation in thepresence of ammonia. In addition, the zeolite used in the catalyst ofthe invention resists crystal collapse under ammonia-rich hydrocrackingconditions and is therefore ammonia-stable. Further still, due to itsmethod of preparation, wherein a Y zeolite is calcined, preferably by asteam calcination, between rare earth cation and ammonium ion exchanges,the zeolite of the invention is hydrothermally stable, resisting crystalcollapse at elevated temperatures in the presence of water vapor, andparticularly under the hydrothermal conditions prevailing under usualhydrocracking conditions.

Although the invention has been described in conjunction with acomparative example and by reference to several embodiments of theinvention, including a preferred embodiment, it is evident that manyalterations, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended in the invention to embrace these and all suchalternatives, modifications, and variations as may fall within thespirit and scope of the appended claims.

I claim:
 1. A zeolite produced by a method comprising cation-exchanging a crystalline aluminosilicate zeolite of the Y type with rare earth-containing cations, calcining the resultant rare earth-exchanged zeolite in the presence of water vapor at a partial pressure of at least about 0.2 p.s.i.a., ion-exchanging the calcined product with ammonium ions, and subsequently ion-exchanging with Group VIII metal cations.
 2. A zeolite produced by a method comprising cation-exchanging a crystalline aluminosilicate zeolite of the Y type with rare earth-containing cations, calcining the resultant rare earth-exchanged zeolite in the presence of water vapor at a partial pressure of at least about 0.2 p.s.i.a., ion-exchanging the calcined product with ammonium ions followed by calcining, and subsequently ion-exchanging with Group VIII metal cations.
 3. A zeolite produced by a method comprising cation-exchanging a crystalline aluminosilicate zeolite of the Y type with rare earth-containing cations, calcining the resultant zeolite in the presence of water vapor at a partial pressure of at least about 0.2 p.s.i.a., ion-exchanging the calcining product with ammonium ions, and subsequently ion exchanging with cations containing a noble metal.
 4. A zeolite as defined in claim 3 wherein said zeolite initially is a sodium Y zeolite and said calcining is accomplished in the presence of water vapor at a partial pressure above 1.0 p.s.i.a.
 5. A zeolite as defined in claim 4 wherein after said noble metal cation exchange, the zeolite contains less than about 0.05 percent by weight of sodium, calculated as Na₂ O.
 6. A zeolite as defined in claim 4 or 5 wherein said noble metal is palladium or platinum.
 7. A zeolite as defined in claim 4 or 5 wherein the zeolite, after said noble metal exchange, contains at least about 2 percent by weight of rare earth metals, calculated as RE₂ O₃.
 8. A zeolite as defined in claim 4 wherein said calcining is carried out at a temperature between about 1100° and 1350° F. and a water vapor partial pressure above about 12 p.s.i.a.
 9. A zeolite as defined in claim 8 wherein, after said ammonium ion exchange, the zeolite is exchanged with noble metal cations, with the resulting zeolite containing at least about 2.0 percent by weight rare earth metals, calculated as RE₂ O₃, less than about 0.5 percent by weight of sodium, calculated as Na₂ O, and at least about 0.1 percent by weight of total noble metals, calculated as the metals.
 10. A zeolite containing a rare earth metal and a noble metal prepared by a method comprising cation exchanging a sodium Y zeolite with cations containing one or more rare earth metals but retaining at least some residual sodium ions, calcining the resultant rare earth-exchanged zeolite in thc presence of water vapor at a partial pressure of at least about 0.2 p.s.i.a. and at a temperature between about 600° and 1600° F. so as to result in at least some shrinkage of the unit cell size of the zeolite, ion exchanging the resultant zeolite with ammonium ions so as to reduce the residual sodium content thereof to below about 0.5 percent by weight, calculated as Na₂ O, and subsequently ion-exchanging with cations containing a noble metal.
 11. A zeolite as defined in claim 10 wherein the unit cell size of said zeolite after said calcination is between about 24.20 and 24.64 Å.
 12. A zeolite as defined in claim 10 wherein the unit cell size of said zeolite after said calcination is between about 24.40 and 24.64 Å.
 13. A zeolite as defined in claim 12 wherein at least 25% of the residual sodium ions are removed by ion exchange with said ammonium ions.
 14. A zeolite as defined in claim 10, 12, or 13 wherein said noble metal is platinum or palladium.
 15. A zeolite as defined in claim 12 wherein said zeolite, after said ammonium ion exchange, contains less than about 0.5 percent by weight sodium, calculated as Na₂ O.
 16. A zeolite as defined in claim 12 wherein said zeolite, after said ammonium exchange but prior to said noble metal exchange, is calcined.
 17. A zeolite as defined in claim 15 wherein said calcination is carried out in the presence of water vapor at a partial pressure between about 2 and 15 p.s.i.a.
 18. A zeolite as defined in claim 17 wherein said zeolite after said ammonium ion exchange contains less than 0.05 percent by weight sodium.
 19. A zeolite as defined in claim 12, 16, or 17 wherein said zeolite after said ammonium ion exchange contains at least 5 percent by weight of rare earth metals, calculated as RE₂ O₃.
 20. A zeolite as defined in claim 17 wherein said zeolite after said rare earth cation exchange contains more than about 1 percent by weight sodium.
 21. A catalyst useful in hydrocarbon conversion reactions involving hydrogenation, said catalyst being prepared by admixing the zeolite of claim 1, 2, 3, 4, 10, 12, or 16 with a refractory oxide component and calcining.
 22. A catalyst useful in hydrocracking prepared by admixing the zeolite of claim 14 with a refractory oxide component and calcining.
 23. A hydrocracking catalyst comprising a zeolite containing a rare earth metal, Group VIII metal, and hydrogen ions, said catalyst being prepared by a method including the steps of cation exchanging a Y-type zeolite with rare earth-containing cations, calcining the resultant rare earth-exchanged zeolite in the presence of water vapor at a partial pressure of at least 0.2 p.s.i.a., ion-exchanging the calcined product with ammonium ions, subsequently admixing with a refractory oxide component, calcining the resulting admixture so as to convert at least some of the ammonium ions to hydrogen ions, impregnating the calcined admixture with a solution containing Group VIII metal cations under conditions such that at least some Group VIII metal cations are exchanged into the zeolite contained in the calcined admixture, and again calcining.
 24. A hydrocracking catalyst prepared by a method including the steps of:(1) cation exchanging a sodium Y zeolite with rare earth cations such that the resultant zeolite contains at least about 2 percent by weight of rare earth metals, calculated as RE₂ O₃, but still retains at least some sodium; (2) calcining the resultant rare earth-sodium Y zeolite in the presence of water vapor at a partial pressure of at least about 0.2 p.s.i.a. so as to effect at least some reduction in the unit cell size of said zeolite, with the resultant unit cell size being between 24.40 and 24.64 Å; (3) ammonium ion exchanging the calcined zeolite from step (2) so as to further reduce the sodium content thereof; (4) ion-exchanging the ammonium-exchanged zeolite from step (3) with cations containing a noble metal; and (5) admixing the zeolite from step (4) with a refractory oxide component and calcining the resultant admixture.
 25. A catalyst as defined in claim 24 wherein the zeolite in the calcined product of step (5) contains at least 2 weight percent of rare earth metals, calculated as RE₂ O₃, and at least 0.1 weight percent of one or more noble metals, calculated as the metals.
 26. A catalyst as defined in claim 25 wherein said zeolite after said rare earth cation exchange in step (1) contains no more than 5 weight percent by weight of sodium but the ammonium-exchanged zeolite from step (3) contains less than 0.5 weight percent sodium.
 27. A catalyst as defined in claim 26 wherein the rare earth-sodium Y zeolite produced in step (1) contains between 1 and 5 percent by weight of sodium, calculated as Na₂ O, and the zeolite in the calcined product of step (5) contains at least 5 weight percent rare earth metals, calculated as RE₂ O₃.
 28. A catalyst as defined in claim 24 or 27 wherein said calcination in step (2) is accomplished in the presence of an atmosphere consisting essentially of steam.
 29. A catalyst as defined in claim 24, 25, or 27 wherein the noble metal exchanged into said zeolite in step (4) is palladium and said calcining in step (2) is accomplished in the presence of water vapor at a partial pressure above about 12 p.s.i.a.
 30. A method for preparing an ammonia-stable and hydrothermally stable zeolite comprising cation exchanging a crystalline aluminosilicate zeolite of the Y type with cations containing a rare earth metal, calcining the resultant rare earth exchanged zeolite in the presence of water vapor at a partial pressure of at least 0.2 p.s.i.a., ion-exchanging the calcined product with ammonium ions, and subsequently ion-exchanging with cations of a Group VIII metal.
 31. A method as defined in claim 30 wherein said calcining results in some shrinkage of the unit cell size of the zeolite.
 32. A method as defined in claim 31 wherein said Group VIII metal is a noble metal.
 33. A zeolite as defined in claim 3 wherein said calcining is carried out in the presence of water vapor at a partial pressure between about 2 and 15 p.s.i.a. under conditions such that the unit cell size of the zeolite undergoes some shrinkage.
 34. A zeolite as defined in claim 33 wherein said water vapor partial pressure during said calcining is between 5 and 15 p.s.i.a., and said noble metal is platinum or palladium.
 35. A zeolite as defined in claim 34 wherein said zeolite contains, due to said exchange wth the rare earth cations, at least 5 percent by weight of rare earth metals, calculated as RE₂ O₃.
 36. A zeolite as defined in claim 35 wherein said zeolite contains less than about 0.5 percent by weight of sodium, calculated as Na₂ O, after said noble metal exchange.
 37. A zeolite as defined in claim 36 wherein said calcining is carried out at a temperature of about 1100° to 1350° F. at essentially atmospheric pressure in the presence of water vapor at a partial pressure above about 12 p.s.i.a.
 38. A zeolite as defined in claim 1, 2, 3, 10, or 34 wherein the rare earth cation exchange introduces a mixture of rare earth cations into said zeolite, said mixture containing cerium, lanthanum, praeseodymium, and neodymium cations.
 39. A catalyst as defined in claim 23, 24, or 27 wherein the rare earth cation exchange introduces a mixture of rare earth cations into said zeolite, said mixture containing cerium, lanthanum, praeseodymium, and neodymium cations.
 40. A zeolite as defined in claim 37 wherein the rare earth cation exchange introduces a mixture of rare earth cations into said zeolite, said mixture containing cerium, lanthanum, praeseodymium, and neodymium cations.
 41. A zeolite as defined in claim 40 wherein said noble metal comprises palladium.
 42. A zeolite as defined in claim 34, 35, 37, 40, or 41 wherein said zeolite contains less than about 0.05 percent by weight of sodium, calculated as Na₂ O, after said noble metal exchange. 